The broad objectives of this research are to understand the relationships among perceived fatigue, aerobic capacity, and physical disability and to provide a scientific basis for evaluation and treatment of aerobic insufficiency and disability in systemic lupus erythematosus (SLE). Debilitating fatigue is among the most common symptoms of SLE. In conjunction with substantial perceived fatigue, physical activity intolerance and low aerobic capacity have been identified in a group of women with mild SLE. The hypothesis that will be investigated is that in SLE, aerobic insufficiency is a modulator of physical durability. The specific aims of this research are to determine the effect of changes in aerobic capacity on perceived fatigue in women with SLE and to determine the influence of aerobic capacity on activity tolerance in women with SLE. Four experiments will be conducted to address these specific aims. Aerobic capacity increases and decreases with exercise conditioning. In all of the experiments, 32 women with SLE will be subjected to a perturbation regimen consisting of an eight-week pre-conditioning (control) period, followed by an eight-week aerobic conditioning period, then an eight-week de-conditioning period. Experiment 1 will assess perceived fatigue scale scores, across the perturbation protocol, with the expectation that perceived fatigue will increase in the conditioned state and decrease in the de-conditioned state. Experiments 2-4 will assess activity tolerance measures during peak and submaximal treadmill tests and will be assessed across the perturbation protocol, with the expectation that activity tolerance will improve in the conditioned state and diminish in the de-conditioned state. Excessive fatigue is disabling in up to 80 percent of those with SLE. This research holds promise for improved evaluation and treatment of SLE fatigue and may have implications for adherence to the Americans with Disabilities Act.